1. Field of the Invention
The present invention is a single step process for the formation of vinyl chloride polymers by emulsion polymerization in an aqueous reaction medium using a water soluble initiator and a mixed emulsifier system.
2. Description of the Prior Art
The use of a single step, or non-seed growth, process for the emulsion polymerization of vinyl chloride using a water-soluble initiator and a mixed emulsifier system is known. For example, in pending U.S. Ser. No. 740,990 of Paul Kraft, it is taught that such a system can be used to yield a resin product that is suitable for use as a plastisol or organosol resin without the need to use conventional seed polymerization procedures. In conventional seed polymerization (such as shown in U.S. Pat. Nos. 3,642,740 and 3,755,225), the product is formed by a two-step process involving: (1) an initial seed formation step; followed by (2) a seed growth step, wherein additional monomer is polymerized in the presence of the previously formed seed particles.
Also, in pending U.S. Ser. No. 740,989, in the name of Donald F. Anderson, an improved single step emulsion polymerization process using a mixed emulsifier system is described. In this process, a mixed emulsifier similar to the one shown in Kraft U.S. Ser. No. 740,990, is present in the reaction medium, when the polymerization reaction is initiated, as a prehomogenized or premixed solution to yield a more reproducible process and one which yields a latex of polymer particles having better mechanical stability. The latex which is produced in this improved process contains a greater proportion of polymer particles having a size in the desired range of from about 0.6 to about 0.7 microns which, when dissolved in plasticizer, also yields a plastisol or organosol having superior physical properties in regard to the stability of the resin to mixing forces. It is desirable in commercial operations that any latex be resistant to coagulation of the polymer particles when mixing forces are applied thereto. Coagulation causes undesired inhomogenity in the latex which adversely affects the flowability characteristics of such solutions. Also, when the resin product is added to plasticizer to form a plastisol or organosol, it is desirable for such solutions to exhibit low viscosity characteristics so that they can be easily manipulated in commercial operations.
Although both of the aforementioned pending applications describe single step (or "one shot") procedures which have superiority over conventional two-step seed polymerization techniques, it has been found that polymerization using such single step processes, under certain temperature conditions, can lead to a deterioration in the resulting latex. For example, lowering of the polymerization temperature from about 60.degree. C. to about 45.degree. C. can, under certain circumstances, result in a deterioration of the quality of the latex when the aforementioned one-step, mixed emulsifier systems are employed.